The helicopter system is characterized by a rotor, which consists of a plurality of rotor blades and rotates at a defined speed. The individual rotor blades in this case have a specific profile and are also arranged on the rotor head of the rotor rotatably about their longitudinal axis, in such a way that a lift is generated by the cooperation between the rotor blade profile and the angle of incidence during the revolution and is similar to that of fixed-wing aircraft.
Vertical air movements are induced within the rotor plane due to the lift within the rotor plane caused by the rotor revolution. In the normal situation, a passage of air through the rotor from top to bottom is induced. In other words, the air is pressed downwardly through the rotor due to the lift that is generated by the rotating rotor and thus provides the helicopter with its flight characteristic, and therefore a vertical air movement, also referred to as downwash, is encountered in this case.
The rotor can also produce lift when air flows against it from below. This is what is known as “autorotation” or the “windmill state”, which is used for example in the event of a power unit failure. The pitch angle of the rotor blades is then reduced and the individual blades generate their own propulsion, just as a glider can fly without drive. In this case, air flows against the rotor from below. The airflow is braked slightly by the rotor, which generates a force upwards. The downwash is then above the rotor.
A dangerous flight condition may occur between the normal situation and autorotation if the helicopter falls in its own downwash due to its high descent rate and the very slow forward flight, this flight condition being known as the vortex ring state (VRS). In this case, ring vortices form at the tips of the rotor blades in such a way that the air masses pushed downwardly are sucked in again above the rotor. The downwardly directed air cannot escape quickly enough, and therefore the lift stops suddenly, such that the helicopter is uncontrollable to some extent and rapidly loses altitude. Up to 100 m of altitude are typically lost in the event of such a critical flight maneuver until the helicopter can be intercepted again accordingly. This has fatal consequences, particularly when close to ground level, with helicopters being suitable for flight at this height due to their system characteristics in particular. FIG. 1 shows an example of such a vortex ring state.
A problem is that the prediction of such a vortex ring state is highly inaccurate since the creation of the vortex is dependent on many factors. On the one hand there must be a correspondingly high descent rate, and on the other hand there must be a correspondingly slow forward flight or perhaps even no forward flight. In addition, the moment at which the vortex ring state is entered may be dependent on a range of ambient factors, which make the prediction process very imprecise.
A further problem, which is caused by the rotor downwash, is the highly inaccurate measurements of the relative movement of the air masses surrounding the helicopter. Sensors that detect a relative movement of the air masses surrounding an aircraft measure the dynamic pressure present and can thus establish the speed of an aircraft compared to the surrounding air. Such a sensor is known for example in the form of the pitot tube. The variometer is generally used in this case for changes in altitude and measures the change in speed of the air pressure dependent on the altitude.
All of these sensors have the disadvantage in this instance that they measure the movement relative to the air surrounding the aircraft, which is only possible to a limited extent within the downwash (beneath the rotor) of a helicopter rotor. This is because the downwash induced from the rotor lift falsifies the measurement of the relative speed of the surrounding air masses and therefore provides only an approximate estimation of such a speed.
Due to the inaccuracies of the sensors and the risk of the vortex ring state, helicopter pilots are currently trained to fly round the critical flight regions with the greatest safety margin possible. However, this leads to uneconomical flights in many areas, which is becoming increasingly significant in current times.